What app can you not live without?

Since there are so many great apps out there, I want to ask the ever-so-important question: Which one is absolutely essential to your daily life?

I'll go first: Waterllama! I started using it a few months ago because I really like to know precisely what and how much I drink each day, especially during the summer.

I'd love to hear about the app you use the most and why it's so important to you. And if you have any tips or tricks for getting the most out of the app, I'd love to hear those too!